The present invention relates to a rubber composition for a chafer.
In general, a belt part (also referred to as a breaker) configuring a tire and a bead part contacting a rim play an important role in the durability of a tire used in a truck or a bus, and a hard rubber (below, also referred to as a chafer) is used to suppress deformation of the rubber. It is desirable to make the rubber hard to suppress the deformation of the rubber. However, because the tip of the chafer breaks during mounting and demounting of the rim if the rubber is made to be too hard, the hardness is generally set to 70 to 75. However, in order to improve the durability of the bead part in a use condition such as a large load, a hardness of at least 76 becomes necessary.
In order to improve the rubber hardness, the rubber composition is made hard by compounding a reinforcing filler such as a carbon black, and a method of improving the hardness by making the rubber component a specified component is used. For example, as in Patent Document 1, it becomes possible to exhibit a hardness of at least 75 by making the butadiene rubber that is used a polybutadiene that contains a 1,2-syndiotactic crystal. However, it is difficult to obtain good fuel efficiency and to suppress a change in physical properties with time even when using these techniques.
Further, a normal bead structure is that the bead is sandwiched between a bead wire (iron) and a lever roller (iron), and a canvas chafer covers the vicinity around the wire. However, a rubber chafer is known in which fiber reinforcement by canvas is not used in order to improve productivity and reduce the weight. Incidentally, in the case of using a blend of the rubber components in Patent Document 1 as the rubber chafer, it is difficult to improve the elongation at break, it was impossible to remove the canvas chafer, and it is difficult to improve the productivity and reduce the weight.
Further, a rubber composition having low elongation at break has a problem that the lever during rim assembly breaks the rubber around a bead toe, and in general, in order to improve the elongation at break, a method such as making the carbon black into fine particles or increasing the content of natural rubber can be considered. However, there is a problem that the low heat build-up property deteriorates.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2006-63143